Combine harvesters are agricultural machines that harvest cereal grain crops, such as wheat, oats, rye, barley, corn, soybeans and flax. Grain and straw are separated in a combine harvester, e.g. by performing the operations of reaping, threshing and winnowing in a single operational process. In a conventional combine harvester, a header cuts the crops and feeds them to a threshing drum, onto which rasp bars are affixed. Following the separation process, waste straw is supplied to a chopper for shredding the straw. The shredded straw may then be ejected downstream the chopper, e.g. it may be spread out over the field. The fundamental integration of a chopper into the material flow of a combine harvester is for example described in U.S. Pat. No. 5,501,635.
The materials supplied to the chopper can be a type of normal dry straw, for example wheat, barley, rye or oats straw. Likewise, corn, sunflowers and/or other plants can also be shredded. In known straw choppers, several chopper knives are arranged on a rotor, whose longitudinal axis is oriented transverse to the direction of travel and lies in a substantially horizontal plane. The chopper furthermore comprises counter-knives, which are non-rotating, for example fixed, with respect to the rotor. The chopper knives are led past the counter-knives at a predefined distance thereof, such that the straw can be cut up between chopper knives and counter-knives.
During operation of the chopper knives, the cutting edges wear out, which may lead to a deterioration of the chopper operation, a changed flow of the straw, reduced flow velocities and, at worst, a blockage of the chopper.
Different variations of chopper knives are known in the art. For instance, a chopper knife can have two opposite cutting edges. This has the advantage that the knife can be inverted in case of wearing-out of a cutting edge oriented in the direction of rotation, thereby roughly doubling its lifespan.
To reduce the wear, the cutting edges of the chopper knives may be hardened. In one example this may be performed by coating them for example with a tungsten carbide or chromium carbide coating. In another example, hard metal plates, e.g. metal carbide plates, may be soldered to the cutting edge of the chopper knives. Both application of a hard layer by coating or by soldering significantly increases the price of the chopper knives, in particular because of the price of the material to be applied.
DE 20 2006 017 540 U1 discloses a coating for chopper knives wherein the thickness of the coating is varied along the direction of the cutting edge. By increasing the coating thickness towards the end of the knife where the edge is expected to wear out faster, the life-expectancy of the chopper knife may be increased. However, application of a coating with variable thickness involves complex and costly manufacturing methods. Furthermore, still more expensive coating material is required.